Sintered magnets prepared by sintering ferrite powder have been known as the permanent magnet. They have been used for various applications. Further, ferromagnetic inter-metallic compounds containing as major constituent elements rare-earth metals and iron group metals, such as samarium-cobalt magnet (hereinafter simply referred to as the rare-earth inter-metallic compound) have been developed in recent years. (See Proceedings of the Eighth International Workshop on Rare-Earth Magnets and Their Applications, Dayton, Ohio, USA, May 1985, 6-8 Edited by Karl J. Strnat.)
Although they have high magnetic performance, the sintered magnets, however, are hard and brittle in themselves, so that they are poor in moldability and have a problem in the dimensional accuracy. Accordingly, so-called plastic magnets prepared by mixing and dispersing magnetic powder in an organic resin (hereinafter simply referred as resin) and molding thus obtained mixture have been developed. The magnetic powder used for this purpose has been mainly composed of ferrite, but, since the magnetic force of such resin-bonded magnets is poor as compared with that of the sintered magnets, development has been made recently to such a resin-bonded magnet using powder of ferromagnetic rare-earth inter-metallic compound as described in Japanese Patent Open-Laid Applications Nos. 49-3196/1974, 50-143765/1975 and 54-16698/1979.
In this specification, the power of ferromagnetic rare-earth inter-metallic powder is referred to as the rare-earth magnet powder.
By the way, with the extending trend for the application uses of those equipments using plastic magnets in recent years, conditions for using them tend to become severer and, particularly, it has highly been demanded to supply resin-bonded magnets excellent in the dimensional stability at high temperature, protectability against water, oil-resistance and solvent-resistance.
Characteristics of the resin-bonded magnets are of course varied depending on the compositions of the starting magnetic powder, kinds of resin as the binder, shapes of the molding product and the like. From the overall point of view, the performance of the resin as the binder is most important so that the magnet may be excellent in the moldability, it may maintain the dimensional stability and magnetic properties during manufacturing steps and that it may have dimensional stability at high temperature, protectability against water, oil resistance and solvent resistance as a shaped substance.
As the resin for the resin-bonded magnets, thermoplastic resins such as polyamide and polyolefin (Japanese Patent Publication No. 59-5218/1984) or thermo-setting resins such as epoxy and phenol (Japanese Patent Open-Laid Application No. 54-16698/1979) have heretofore been used. They are used, in view of the magnetic property and the physical strength, in an amount within a range usually from 55 to 12 % by volume (about 15-2 % by weight). They, however, have a heat expansion coefficient as high as about 5-15.times.10.sup.-5 1/.degree. C., so that they have poor dimensional stability at high temperature. Further, although there have been those resins excellent in oil resistance and solvent resistance, they are poor in dimensional stability at high temperature and protectability against water.